1. Field of the Invention
The invention is directed to a device for measuring the mass of a flowing medium, or flow rate meter, with a sensor carrier for holding a measuring element, and on a method for producing a device for measuring the mass of a flowing medium with a sensor carrier.
2. Description of the Prior Art
German Patent DE 44 26 102 C2 and U.S. Pat. No. 5,693,879, disclose a sensor carrier for a measuring element in an air flow rate meter in which the sensor carrier with the measuring element protrudes into a measuring conduit in which a medium flows. The measuring element furnishes a measurement signal, which serves to calculate the mass of the flowing medium.
The sensor carrier has a recess, into which the measuring element is placed flush and retained by means of an adhesive layer applied to a bottom face of the recess. The sensor carrier is produced by first making an opening in a metal strip, the opening corresponding approximately to the external shape of the measuring element, and after that the metal strip is bent around a bending axis outside the recess and then compressed such that a bent portion of the metal strip forms a retaining element, and an unbent portion of the metal strip having the opening forms a frame element of the sensor carrier. The retaining element covers the opening of the frame element and with it forms a recess. After that, by further reshaping operations of the retaining element, mesa-like protuberances are created, which act as spacers or as a bearing face. The measuring element is then glued into the recess.
It is extraordinarily important that the measuring element with its surface be glued as flush as possible to the surface of the sensor carrier into the recess, since even the slightest offset, for instance from an unevenly applied adhesive layer, leads to eddies and separation zones that adversely affect heat dissipation from the measuring resistor, particularly at the surface of the measuring element, and adulterate the outcome of measurement. Very slight measurement tolerances in the recess must therefore be provided, and when the measuring element is glued into the recess of the sensor carrier, extreme care is needed; particularly in mass production of the device, this entails a major engineering effort, leading to high production costs.
The various work steps in producing the frame and retaining element are a disadvantage. The flowing medium can additionally flow through the seam gap between the frame and retaining element. However, this is not disadvantageous, since this effect can be suppressed by zero point measurement and calibration. However, the outcome of measurement is adulterated during the service life of the measuring element if this seam gap becomes plugged with dirt particles and/or liquid and the calibration is no longer correct.
It is disadvantageous that the spacers are not formed until a further shaping process. The tolerance in terms of the depth of the recess is dictated by the tolerance in the thickness of the metal strip and the tolerance of the seam gap thickness.
It is also disadvantageous that because of the flowing corrosive medium, a corrosionproofing layer must be applied to the sensor carrier, such as NiNiP, by means of an additional, expensive electroplating process or a coating method, and this layer further increases the dimensional processes, production times, and production costs.
In this kind of self-supported way of fastening the measuring element, tolerances in production create a gap between the measuring element and the recess of the sensor carrier. The gap can be so large that an undesired flow under the hollow chamber beneath the diaphragm of the measuring element can occur in the recess, which adversely affects the measurement outcome of the device.
In the literature, devices have therefore been described in which the disruptive influence of the flow underneath can be reduced.
Diverting the flow at a specially shaped edge of the measuring element, as described in German Patent Disclosure DE 195 24 634 A1 and U.S. Pat. No. 5,723,784, prevents the medium flowing in via the gap from reaching a hollow chamber underneath the diaphragm of the measuring element.
Applying adhesive seams, as described in German Patent Disclosure DE 197 43 409 A1, can prevent the penetration of the medium into the gap around the measuring element in order to prevent unwanted flows underneath.
However, a disadvantage of both methods is that only by the special provision of the adhesive seams or by additional provisions is the flow deflected around the hollow chamber in order to compensate for the affects of the production tolerances.
German Patent Disclosure DE 197 44 997 A1 discloses a device which makes it possible to protect the components of an evaluation circuit and the connecting lines to the contacting region of the measuring element from moisture by means of a gel, and soiling of the sensor region, that is, the part of the measuring element where a diaphragm is located, by the gel is prevented. Widening of a gap that extends between the measuring element and the walls of the recess is provided in various places, so that by means of the widened places, a further flow of a protective layer, applied at least in part to the evaluation circuit, in the gap can be reliably stopped, so that the flow path of the protective layer always remains unambiguously defined. This has disadvantages in terms of production, because gaps must additionally be created, and the flow of gel is not stopped but merely deflected in a defined way.
The device of the invention and the method of the invention have the advantage over the prior art that in a simple way, production and a measurement outcome even over a relatively long operating time are improved, since there is no longer any flow under the measuring element, since by narrowing the tolerances by the use of plastic, very precise placement of the measuring element into the sensor cavern is possible. Because of the arbitrary possibilities for shaping the plastic, it is possible to achieve filigreed shapes and take aerodynamic requirements, such as for the leading edge, into account, which cannot be done with metal. Since plastic in comparison with metal does not corrode as severely, no further corrosionproofing is necessary.
The production-dictated variation in the measurement outcome in the prior art resulting from an air flow through the open seam gap, or a seam gap that is becoming plugged, does not exist, and the tolerance in terms of the depth of the recess is determined according to the invention only by the tolerance of the shaping tool and no longer by the tolerance of the seam gap as well.
It is especially advantageous to produce a sheet-metal frame as a sheet-metal element and to spray-coat the sheet-metal frame; during the spraying process, the sheet-metal frame is firmly held in the shaping tool.
It is advantageous if the plastic used is selected from the class of plastics of liquid crystal polymers or partially crystalline aromatic thermoplastic.
It is also advantageous that the bulkhead is sprayed-on, because this makes an adhesive bond between the bulkhead and the sensor carrier unnecessary.
In the assembly process, it is advantageous that a bead of adhesive is placed crosswise over the sensor cavern bottom in the recesses in the longitudinal edges of the sensor cavern and completely seals off the sensor region of the measuring element in the sensor cavern, and that indentations are made in the peripheral region of the sensor cavern bottom, so that the measuring element can be inserted more precisely. Soiling of the measuring element is prevented by this bead of adhesive, by reliably stopping the gel that protects an evaluation circuit against moisture.
In the production, it is advantageous for the plastic for the plastic element to be applied to the sheet-metal element by such methods as eddy sintering or immersion or electrostatic powder spraying, and for the plastic element to be shaped after that.